In an endeavour to render premises and property generally more resistant to theft, various forms of alarm device have been put forward, which operate on the principle of an alarm being given when an alarm-triggering member is moved from a given set position. Movement of the alarm-triggering member opens or closes a circuit connected to connecting contacts for triggering an alarm signal. An electrical contact making arrangement of that kind is disclosed for example in German patent specification No. 550 611, being used in a frame bar of an electrical protection grid or grating which is installed to protect premises from being broken into. The frame bar is made up of two halves and thus forms an outside conductor, with the two halves being secured together by means of screws. An inside conductor is formed by a bar member which is carried by one of the two halves of the frame bar. Disposed between the bar member and the halves of the frame bar are spacer sleeves of insulating material, through which pass screws which are prevented from contacting the bar member by means of insulating discs and are screwed into the halves of the frame bar. At given spacings the bar member has openings through which screws are passed. The diameter of those openings is approximately equal to the outside screwthread diameter of the screws passing therethrough so that the screwthread portion of each screw can still just be passed through the respective opening. That part of the shank of the screw which lies in the opening is provided with a peripheral groove so that between the shank of the screw and the bar member there is a sufficient clearance to prevent current from passing between the shank of the screw and the bar. If an attempt is made to remove the frame by taking it apart, then the screws, on being released, bridge the inside conductor formed by the bar member to the outside conductor formed by the frame bar, and an alarm signal is triggered off as a result.
However, that known construction is relatively expensive and complicated and further suffers from the disadvantage that parts thereof which carry current for the purposes of triggering off the alarm signal are readily accessible from the outside so that it is comparatively easy for the arrangement to be rendered inoperative.
German patent specification No. 384 191 discloses a switching means for triggering off an electrical alarm installation in the event of attempted theft of carpets, wall coverings and similar items in which the alarm contacts are covered by the covering material which is to be protected. The control members for the masked alarm contacts are control members which simulate carpet nails or carpet screws and which are not externally distinguishable from the carpet nails or carpet screws which are actually used for fixing the article in position. In the switching arrangement, the pin portion of the nail or the shank portion of the screw, on the head of the nail or screw respectively, is in the form of a switch rod around which is wound a coil spring for holding apart two alarm or connecting contacts which are connected to an electrical circuit. When the attempt is made to pull the nail or screw out, the switch rod on the head of the nail or screw compresses the spring and then the two alarm contacts also come into contact with each other, thereby triggering off the alarm signal.
That arrangement is also of a relatively complicated construction, together with the fact that, when the switch rod which is subjected to the loading of the spiral spring is pulled out, that loading means that the force required to pull out what is apparently a fixing nail or screw is different from the force required to pull out the normal fixing nails or screws. As a certain spacing must be provided between the two alarm contacts in order to ensure that the warning installation is not accidentally triggered off by the vibration due to the footsteps of someone walking on the floor, or the like, it is possible by carefully pulling out what is apparently a fixing nail or screw to ascertain whether it is or is not in fact a fixing screw or nail which serves only for fixing the article in position. In addition, there are no means for preventing that the switch rod and the head thereof are not current-carrying members while the alarm signal is being given.